This invention relates to a semiconductor device which has a semiconductor element sealed by a synthetic resin (will be simply called a resin hereinunder) and mounted on a chip carrier, such as a tape carrier.
As a rule, it is a recent trend that a semiconductor device becomes small in size, thin in thickness, and light in weight. In order to be matched with this trend, automation has been made also as regards an assembling process of mounting a semiconductor element or chip onto a package which has a plurality of wiring patterns or outer leads on a package surface. Since the semiconductor element is mounted or attached onto the package surface of the package, the above-mentioned assembling process is referred to as a surface mounting method in the art. The surface mounting method has been usually adopted in a semiconductor device of a Quad Flat Package (QFP) structure or a Small Outline J-leaded Package (SOJ) structure.
Herein, it is to be noted that the outer leads are located along an outer periphery of the package and are one-dimensionally extended and that they are electrically connected to electrodes of the semiconductor elements by the use of conductive wires one by one.
With this structure, a spacing or a pitch between two adjacent ones of the electrodes of the semiconductor element becomes narrow as the electrodes are increased in number. Such a narrow pitch makes a mounting process difficult when the pitch becomes narrow over a limit of a mounting technique.
On the other hand, a semiconductor device has been proposed in U.S. Pat. No. 5,216,278 issued to Paul T. Lin et al and has a package structure known as a Ball Grid Array (BGA). With this structure, it is possible to derive wiring patterns two-dimensionally from the electrodes of the semiconductor element arranged two-dimensionally in the form of pads on the semiconductor element.
More specifically, the package structure comprises a carrier substrate of a flexible material, such as a resin material, which has a die attach surface and a package mounting surface opposite to the die attach surface. A semiconductor element which has a plurality of pads is bonded to the die attach surface through a conductive layer. Any other package leads are arranged around the semiconductor element on the die attach surface and are electrically connected to the pads on the semiconductor element through conductive lines. Some of the package leads are electrically connected through via holes to package leads formed on the package mounting surface.
A plurality of solder balls are placed on the package leads on the package mounting surface to be attached to a printed circuit board. Thus, a combination of the carrier substrate and the package leads on the die attach surface and the package mounting surface may be collectively called a chip carrier.
Furthermore, the semiconductor element on the die attach surface of the carrier substrate is sealed or molded by a resin material.
With this structure, the resin material is formed only on a side of the die attach surface of the carrier substrate. This shows that the resin material is adhered to the chip carrier only on a side of the die attach surface with a side of the package mounting surface uncovered with the resin material.
Consequently, the resin material is fixed to the chip carrier only by adhesion of the resin material.
Herein, it is to be noted that the resin material is liable to absorb moisture and has a thermal expansion coefficient different from that of the chip carrier. In addition, the carrier substrate itself has a high hygroscopicity.
Moreover, such a semiconductor device of the type described is heated on mounting the semiconductor device onto the printed circuit board so as to melt the solder balls and to attach the semiconductor device to electrodes laid on the printed circuit board.
Under the circumstances, the semiconductor device is disadvantageous in that the resin material is easily peeled from the chip carrier when the semiconductor device is heated on mounting the semiconductor device onto the printed circuit board. This is because evaporation of moisture takes place in the resin material by a heat treatment on mounting the semiconductor device onto the printed circuit board and the resin material is largely expanded in comparison with the chip carrier.
Taking the above into consideration, the resin material is dried within an oven to eliminate the moisture before the semiconductor device is mounted onto the printed circuit board. Thus, the semiconductor device should be deliberately handled in order to prevent the resin material from being peeled from the chip carrier. As a result, the semiconductor device is troublesome in handling.
Furthermore, the semiconductor device is degraded in characteristics and can not be guaranteed in a long term reliability because the semiconductor device is directly bonded on the chip carrier which has a high hygroscopicity and is also not sealed by a resin material.
In addition, since the conductive lines are used to electrically connect the pads of the semiconductor element to the leads, a pitch between two adjacent ones of the pads should be equal to or wider than 120 micron meters. As a result, it is difficult with the above-mentioned method to mount, on the chip carrier, a semiconductor element which has a pitch narrower than 120 micron meters.
The semiconductor device also has disadvantages that bonding strength of connections is weak between the leads and the conductive lines, because the leads are formed by copper which is soft and which can not be heated to a high temperature.
Moreover, long electrical paths are required between the pads of the semiconductor element and the printed circuit board, which results in a long transmission delay time and a large noise.